Estimation of Crystalline Structure and Gas Transport Properties of Crystalline Fluorinated Copolymer

PFA has a unique role in the plastics industry due to its inertness, heat resistance, and low coefficient of friction in a wide temperature range.  Generally speaking, fluorinated compounds and fluoropolymers have excellent chemical resistance, and oil- and water-shedding resistance. They have been used as rubbers at high temperatures and in several lubricating fluorine manufactured products.

Anyway, the crystalline polymers were used in the gas permselective, barrier and separating films. The gas transport properties are related to solubility, diffusivity and permeability in a polymer. In general, gas permeability, diffusivity, and solubility decrease with increasing crystallinity in a polymer film, because of gas molecules cannot diffuse and dissolve in the crystalline region. A few paper reported gas transport properties of PFA. In these papers discussed film thickness, pressure and grade dependence of gas permeability in PFA. However, there has been no systematic research on the relationship between gas transport properties and crystalline structure in PFA.

We have investigated the relationship between amorphous structure and its gas permeability of Poly(tetrafluoroethylene-co-perfluoroethylvinylether) (PFA) films using differential scanning calorimetry (DSC), wide-angle, small-angle X-ray scattering and quasi-elastic neutron scattering measurements. We focused on the hierarchical inter-lamellar amorphous structure of various gas-permeable PLA films.  For controling cooling rate from the molten state, we prepared various crystallinity sample.  The crystallnity range is between 29.3% (melt quenched) and 38.3 % (cooling rate, 0.5 K/min). 

From wide-angle x-ray scattering measurements, the crystal lattice is independent of cooling rate, however, the lamellar crystal structure is strongly dependence on the cooling rate.  The thickness of lamellar crystal and amorphous increase with decreasing cooling rate.  The gas permeability coefficient was almost independent of crystallinity, that is, below crystallinity of 32 %, the gas permeability and diffusivity decreases with increasing crystallinity, while above that of 32%, it is independent of crystallinity.  The order of gas permeability coefficient of all PFA films was the same as that of gas size such as the kinetic diameter: 2.89 Å (H2), 3.30 Å (CO2), 3.46 Å (O2), 3.64 Å (N2), 3.80 Å (CH4).   The gas molecules might pass through the amorphous region of inter-lamellar crystals.  Furthermore, we evaluated the relaxation processes of various crystallnity PFA films with quasi-elastic neutron scattering measurements.  The films has two relaxation mode, 10^-12 s and 10^-13 s.  The longer relaxation time component (~10^-12 s), which was assigned as "relaxation of side chain", has correlated with gas permeability.